Hotels and their towels

You know how every hotel these days has their ecological bathroom sign? The one that tells you that washing hundreds of towels every day with detergent is hell on waterways and the environment, such that if you don't want them to wash your towels, you should hang them up to signal further use?

Trying not to be too environmentally unconscious, I've been hanging them up for years.

Without exception, in every hotel in every country I visit, I've always found new, washed towels sitting nicely on the bar or shelf when I come back later that same day.

Maybe it's too expensive to train the staff to read the sign?